Filter structure

ABSTRACT

A filter structure, comprising a first signal line, a second signal line, a third signal line and a fourth signal line, said first and third signal lines defining an input port and said second and fourth signal lines defining an output port of a section of said filter structure, said section being defined by a first bulk acoustic wave resonator (A) which is connected between said first signal line and said second signal line, a second bulk acoustic wave resonator (G) which is connected between said third signal line and said fourth signal line, a third bulk acoustic wave resonator (C) which is connected between said first signal line and said fourth signal line, and a fourth bulk acoustic wave resonator (E) which is connected between said second signal line and said third signal line; is characterized in that a frequency pulling factor defined d of at least one of said acoustic wave resonators is non-zero, said frequency pulling factor d being defined by for first and second bulk acoustic wave resonators having a resonant frequency fr 1 , and for third and fourth bulk acoustic wave resonators having an anti-resonant frequency fa 2 , f 0  being the centre frequency of the filter structure and k the coupling factor of respective resonators.

The invention relates to a filter structure, comprising a first signalline, a second signal line, a third signal line and a fourth signalline, said first and third signal lines defining an input port and saidsecond and fourth signal lines defining an output port of a section ofsaid filter structure, said section being defined by a first bulkacoustic wave resonator, which is connected between said first signalline and said second signal line, a second bulk acoustic wave resonatorwhich is connected between said third signal line and said fourth signalline, a third bulk acoustic wave resonator which is connected betweensaid first signal line and said fourth signal line, and a fourth bulkacoustic wave resonator which is connected between said second signalline and said third signal line. At least one further section of first,second, third and fourth acoustic wave resonators is provided, whereinthe input port of each said further section is connected with the outputport of a preceding section to form a multiple-section filter structure.

Thin-film bulk acoustic wave filters (BAW filters) promise highlyminiaturized, potentially integratable, front-end selectivity inwireless communication devices, such as second-generation (2G) andthird-generation (3G) handsets. Miniaturization follows from the ordersof magnitude smaller wave length of acoustic waves compared toelectromagnetic waves at given frequency. The required electromechanicalenergy conversion in thin-film BAW filters is provided by apiezoelectric layer whose thickness is inversely proportional to thefilter center frequency which is typically about 1-2 μm at 2 GHz.

Best piezoelectric films to date, in terms of crystallographicorientation and acoustic loss, have been achieved with aluminum nitride(AIN). The excellent properties of this material translate intoimportant front-end filter characteristics such as low pass-bandinsertion loss and spurious-mode free stop band. Membrane andBragg-reflector structures have been investigated as means of confiningthe acoustic energy to the piezoelectric layer. The invention isapplicable to both types of structure.

The principle disadvantage of AIN as a BAW filter material is itsrelatively low piezoelectric coupling coefficient, a parameter whichdetermines the maximum achievable bandwidth.

Conventionally, filters are configured as so-called ladder filters,wherein a plurality of series resonators is connected between the inputand output port of the filter, and are grounded by shunt resonatorsconnected between each pair of series resonators.

A lattice-type filter structure as defined in the introductory isdisclosed in EP 1 017 170 A2, based on combinations of series-connectedand cross-connected thin-film BAW resonators. The filter structure hasvery steep attenuation slope outside the pass band. A similarlattice-type structure is described in U.S. Pat. No. 5,692,279 A.

It is the object of the invention to provide a filter structure withfurther extended bandwidth which can be used in second-generation andthird-generation mobile communication systems.

This object is achieved by a filter structure as defined in claim 1.Preferred embodiments are subject-matter of dependent claims, a mobilecommunication means based on this filter structure is subject-matter ofclaim 6.

According to the invention, a frequency pulling factor defined δ of atleast one of said acoustic wave resonators is non-zero,

-   -   said frequency pulling factor δ being defined by        $\delta = {2{( {\frac{f_{r1}}{f_{0}} - 1} ) \cdot \frac{1}{k^{2}}}}$    -   for first and second bulk acoustic wave resonators having a        resonant frequency f_(r1), and        $\delta = {2{( {1 - \frac{f_{a2}}{f_{0}}} ) \cdot \frac{1}{k^{2}}}}$        -   for third and fourth bulk acoustic wave resonators having an            anti-resonant frequency f_(a2), f₀ being the centre            frequency of the filter structure and, k the coupling factor            of respective resonators. The resonant and anti-resonant            frequencies of resonators are indicated by f_(r) and f_(a),            respectively.

By a combination of lattice architecture and frequency pulling, a widerflat region of the passband is achieved. The addition of extra sectionsresults in a close grouping of additional transmission maxima near theband edges of the lattice design, therefore the presence of more thanone section is crucial to achieving at the inventive aim.

In a preferred embodiment, the frequency pulling factor is positive,further preferred equal to or greater than 0.1 and still furtherpreferred equal to or greater than 0.2. It is also preferred to set thefrequency pulling factor equal to or smaller than 10.0, and stillfurther preferred equal to or smaller than 1.0.

The disadvantage of frequency pulling is that a dip is created in thecenter of the passband. Therefore, in a further preferred embodiment, atleast one inductor is included in any of said first, second, third andfourth signal lines, whereby a nearly flat passband can be restored.

The invention may be used to implement multiplex filters. The 3G WDCMAsystem operates in full duplex mode. If both RX and TX paths areconnected to a single antenna, a duplexer is required at the front-endand to separate the two signals. Currently, a ceramic duplex filter isseen as the solution. An important difference between duplexers andstand-alone filters is that the two component filters which areconnected in parallel at the antenna port, load each other electrically.With a BAW solution based on AIN, both filters appear asdifferent-valued capacitors in each others pass band.

Embodiments of the invention will be described in detail below, by wayof example only, with reference to the accompanying drawings, wherein

FIG. 1 illustrates a schematic circuit of a two-section lattice filter;

FIGS. 2(a) and (b) show responses of single-section ladder and latticefilters with no frequency pulling and no series inductors, wherein theresonator figure of merit is 10 and 40, respectively;

FIGS. 3(a) and (b) show responses of two-section ladder and latticefilters with no frequency pulling and no series inductors, wherein theresonator figures of merit are 10 and 40, respectively;

FIGS. 4(a) and (b) show responses of two-section ladder and latticefilters with frequency-pulling factor of 0.2 and no series inductors,wherein the resonator figures of merit are 10 and 40, respectively;

FIGS. 5(a) and (b) show responses of two-section ladder and latticefilters with frequency-pulling factor of 0.2 and normalized seriesinductor reactance of 0.3, wherein the resonator figures of merit are 10and 40, respectively;

FIGS. 6(a) and (b) show a thin-film BAW resonator implemented as asingle resonator with a via and employing a membrane structure toconfine the energy;

FIGS. 7(a) and (b) illustrate a thin-film BAW resonator implemented astwo identical resonators in series and employing a Bragg-reflectorstructure to confine the energy;

FIG. 8 shows a top metalization layer of twin-lattice filter;

FIG. 9 shows top and bottom metalization layers of twin-lattice filtersviewed from below;

FIG. 10 shows top metalization layer, additional thick metal layer andmass-loading layer of twin-lattice filter with possible connectingpoints indicated by dots;

FIG. 11 shows top and bottom metallization layers of twin-lattice filterwith holes provided to reduce sensitivity to mask misalignment, asviewed from below; and

FIG. 12 is a schematic circuit of a duplexer employing twin-lattice RXand TX filters and a single matching inductor at the antenna port. Thefollowing abbreviations will be used; partly derived from a narrow-bandapproximation:

-   -   f₀ center frequency of the filter    -   f_(r) resonant frequency of a resonator    -   f_(a) anti-resonant frequency of a resonator    -   δ frequency pulling factor    -   k={square root}{square root over ( )}[f_(a) ²/f_(r) ²)−1]        coupling factor of resonators    -   Q quality factor of resonators    -   η=Qk² figure of merit of resonators    -   Ω=2(f−f₀)/(f₀k²) normalized frequency    -   α ratio of reactance of series tuning inductor to termination        resistance at center frequency of filter.

Specifically, resonant and anti-resonant frequencies can be expressed asfollows:

-   -   f_(r1)=(1+0.5δk²)f₀ resonant frequency of series resonators        (both filter types)    -   f_(a1)=(1+0.5δk²) (1+k²)f₀ anti-resonant frequency of series        resonators (both filter types)    -   f_(r2)=(1+0.5δk²)f₀/(1+k²) resonant frequency of shunt (ladder)        or cross resonator (lattice)    -   f_(r1)=(1+0.5δk²)f₀ anti-resonant frequency of shunt (ladder) or        cross resonator (lattice).

The static capacitance C₀ of each of the two types of resonator in eachconfiguration discussed herein are assumed to be equal, with valuesproviding optimum electrical match to the terminating impedance of thefilter.

FIG. 1 illustrates a schematic circuit of a two-section lattice filter.An input port of the filter is defined across nodes 1 and 3, an outputport across nodes 2 and 4. Each section consists of four thin-film BAWresonators A, C, E, G and B, D, F, H, wherein resonators A, G and B, Hare series resonators E, C and D, F are cross resonators. At nodes 5 and6, both sections are connected with another. Omitted in FIG. 1 areseries inductors at the ports.

As it is shown in FIGS. 2(a) and (b), a single lattice sectioncomprising two series resonators and two cross resonators does notitself provide more bandwidth than a corresponding T-section of twoseries and one shunt resonator used in ladder filters. This is trueregardless of any frequency pulling. Its main advantage is that its stopband is limited only by parasitics and not by the static capacitance ofthe resonators themselves. The results, exemplarily shown for the tworesonator figures of merit 10 and 40, corresponding to Q˜200 and Q˜800with k=0.22 and δ=α=0, confirm that a single lattice section is moresymmetrical and provides more out-of-band rejection, but is not wider inits passband than a single ladder-filter T-section.

As it is shown in FIGS. 3(a) and 3(b), if a filter comprises more thanone section and no frequency pulling is introduced, the lattice filterhas wider band width compared to a ladder filter. However, the responseof the lattice filter is not very flat. Also the effect of loss on thepassband responses of ladder and lattice filters is different.

FIGS. 4(a) and (b) show the effect of increasing the separation ofseries and cross-connected resonator frequencies with δ=0.2, but keepingthe normalized series inductor reactance α=0, for otherwise identicaltwo-section filters. The response of the lattice filter is now flatterthan that of the ladder filter, except that it exhibits a morepronounced dip in the band center. This is the disadvantage of frequencypulling.

As shown in FIGS. 5(a) and (b), for small dips, of the order of 1 dB, anearly flat passband can clearly be restored by adding series inductors.The extent to which this is effective dictates the upper limit tobandwidth that can be achieved using the measures proposed. Althoughlarge-value high-Q external inductors are highly undesirable, it isfound that typically only a small value and a low Q-factor are requiredfor the invention. For example, a 1.5 nH inductor with Q of ˜20 may besufficient. This value of series inductor Q increases insertion loss byonly about 0.1 dB compared to an ideal loss-less inductor of the samevalue. This suggest that typically no more than about 1 mm² of space islikely to be required for each inductor, so these components could beintegrated at IC, MCM or PCB level or indeed incorporated as bond-wires.In FIGS. 5(a) and (b) the responses show the ladder filter passbandexhibiting substantial “role-off” at the band edges, whereas the latticefilter passband is seen to be enhanced near the band edges giving anearly flat response, therefore providing substantially more usefulbandwidth. The difference in bandwidth and flatness of response betweenthe two architectures is apparent for both values of resonator Q factor.Therefore, although the absolute insertion loss is decreased withincreasing resonator Q value, the retention of the flatness of thepassband in the presence of loss could in some cases reduce thespecification for the resonators. A filter with two lattice sections,also referred to as twin-lattice design, appears to give the flattestpass band response given optimization of other parameters. Also, sincethe out-of-band rejection provided by a lattice section is intrinsicallymuch greater than that of a ladder T-section, no more than two sectionsare likely to be needed to meet typical out-of-band specifications.

There are two particular advantages of the twin-lattice design comparedto the use of more than two lattice sections. Firstly, since eachsection adds some loss, the overall insertion loss is lower. Secondly, alayout without vias is feasible, as further described below, whereasthis does not appear possible with more than two lattice sections due tothe complexity of the interconnections. The wafer processing cost isthen kept at the same level as that required for thin-film BAW ladderfilters.

Another advantage of lattice filters is that they haveelectrically-balanced ports, compared to the unbalanced ports of theladder filter. It is therefore simple to incorporate balun functionality(balanced-to-unbalanced transformation) in a lattice filter, simply bygrounding one side of one port. This function is typically required inthe receiver (RX) chain of a transceiver between antenna and low-noiseamplifier (LNA).

A thin-film BAW filter comprises sets of interconnected resonatorsformed in a layered structure. FIGS. 6 and 7, respectively, showexamples of membrane and Bragg-reflector resonators in a side view (a)and a top view (b). Both employ a piezoelectric layer 3, typicallyc-axis orientated AIN of the order of 1 μm thick, sandwiched between twometal electrode layers 1, 2 each typically of the order of 0.1 μm thick,to provide the main resonator function. The overlay 4 shown, typicallySiO₂, is for frequency adjustment, where required, provided by itsmechanical loading effect.

FIG. 6 shows a configuration where access to the bottom electrode 2 isprovided by a via 5.

FIG. 7 shows an alternative configuration where the resonator isimplemented as two identical resonators R1, R2 in series with a floatingcentral electrode 2 in the bottom metallization layer. This arrangementavoids the via, but adds the cost of a fourfold increase in area.

Both types of resonator configuration may be implemented in either themembrane or Bragg-reflector structure. When an electrical signal, at thefrequency for which the wave length of the thickness-extensionalacoustic mode is approximately twice the piezoelectric layer thickness,is applied between the two electrode layers, this mode which ischaracterized by alternating extension and compression in the thicknessdirection is strongly excited. Other orientations of the piezoelectriclayer or materials of alternative crystallographic symmetry would giverise to other acoustic modes.

The invention is essentially independent of which of these variousapproaches to resonator design is taken. However, it is important thatthere is the option of a via-free layout of a twin-lattice design.

FIG. 8 shows the layout of the top metallization layer in one embodimentof a twin lattice filter; FIG. 9 shows both top and bottom metallizationpatterns from below. The node and resonator labels correspond to thoseof FIG. 1. The dark area is the top metallization layer, typicallyaluminum (Al), or molybdenum (Mo), and the bright area is the bottommetallization layer, typically platinum (Pt), or Al, or molybdenum (Mo).Resonators are formed by the overlap of these two areas, each resonatorof the schematic being implemented as two physical resonators in series,thus avoiding vias as discussed above.

FIG. 10 shows the top metallization, again in dark, together with a greyarea, which indicates where the mass-loading overlay is deposited toreduce the frequency of the cross-connected resonators, and a dark greyarea where the thickness of the top metallization is increased,typically to 5 μm, in order to reduce the resistance of theinterconnections. Possible locations of contact points 3 a, 3 b, 4 a, 4b are also shown. The two ports are on the left and right ends of thelayout. A contribution to the required series inductance is alsoprovided in this design by the conductors connecting resonators E and Gto nodes 3 a and 3 b, and resonators F and H to nodes 4 a and 4 b. Theoverall dimensions of the pattern shown are about 2.5 mm×1.7 mm, thedesign center frequency is 2.14 GHz.

FIGS. 8 to 10 show that the twin-lattice design can be implementedwithout using vias in the cross-connection, by unfolding the schematiccircuit shown in FIG. 1, so that the crossovers are transferred to theports. This will still leave one terminal at each port inaccessiblewithin the constraint of a planar structure. However, in practice, thethird dimension is likely to be employed at the ports for packaging,either using bond-wires or alternatively solder balls in a flip-chipconfiguration. The two inaccessible terminals then become accessible.The required small inductors could then be implemented as bond-wiresand/or printed connections on the carrier.

FIG. 11 shows top (dark) and bottom (bright) metallization patterns fora variant of the above design. This includes rectangular slots in thetop metallization in locations corresponding to edges of the bottomelectrodes. These have the effect of making the areas of the resonatorsinsensitive to misalignment between the two electrode masks. The use ofenclosed slots or holes ensures that the additional resistanceintroduced into the interconnection paths by this measure is minimal.

FIG. 12 shows a schematic circuit of a duplexer employing twin-latticefilters on the receiving (RX) and transmitting (TX) sides, and a singlematching inductor at the antenna port. The additional bandwidth providedby the invention may allow the filters for the TX and RX paths to employthe same thickness of AIN, the frequency difference being achievedsolely by mask-loading the TX filter, e.g. with an additional silicondioxide (SiO₂) overlay. This reduces the bandwidth of the TX filter, butthe margin in hand provided by the invention makes this feasible. Therewould then be a reduction in both complexity and cost of the thin-filmprocessing, since mass-loading is already required to achieve the lowerfrequencies of the cross-connected resonators, and deposition ofamorphous SiO₂ is much less critical than deposition of highly-orientedAIN.

An important difference between duplexers and stand-alone filters isthat the two component filters which are connected in parallel at theantenna port, load each other electrically. This is compensated by asingle parallel matching inductor at the common port, with the seriesinductors omitted.

1. A filter structure, comprising a first signal line, a second signalline, a third signal line and a fourth signal line, said first and thirdsignal lines defining an input port and said second and fourth signallines defining an output port of a section of said filter structure,said section being defined by a first bulk acoustic wave resonator (A)which is connected between said first signal line and said second signalline, a second bulk acoustic wave resonator (G) which is connectedbetween said third signal line and said fourth signal line, a third bulkacoustic wave resonator (C) which is connected between said first signalline and said fourth signal line, and a fourth bulk acoustic waveresonator (E) which is connected between said second signal line andsaid third signal line; wherein at least one further section of first,second, third and fourth bulk acoustic wave resonators (B, H, F, D) isprovided, wherein the input port of each said further section isconnected with the output port of a preceding section to form amultiple-section filter structure, characterized in that a frequencypulling factor defined δ of at least one of said acoustic waveresonators is non-zero, said frequency pulling factor 6 being defined by$\delta = {2{( {\frac{f_{r1}}{f_{0}} - 1} ) \cdot \frac{1}{k^{2}}}}$for first and second bulk acoustic wave resonators having a resonantfrequency f_(r1), and$\delta = {2{( {1 - \frac{f_{a2}}{f_{0}}} ) \cdot \frac{1}{k^{2}}}}$for third and fourth bulk acoustic wave resonators having ananti-resonant frequency f_(a2),f₀ being the centre frequency of thefilter structure and k the coupling factor of respective resonators. 2.The filter structure of claim 1, characterized in that said frequencypulling factor is positive.
 3. The filter structure of claim 1 or 2,characterized in that said frequency pulling factor is equal to orgreater than 0.1.
 4. The filter structure of claim 1 or 2, characterizedin that the frequency pulling factor is equal to or greater than 0.2. 5.The filter structure of claim 1, characterized in that the frequencypulling factor is equal to or smaller than 10.0
 6. The filter structureof claim 1, characterized in that the frequency pulling factor is equalto or smaller than 1.0
 7. The filter structure of any of claims 1 to 6,characterized in that at least one inductor is included in any of saidfirst, second, third and fourth signal lines.
 8. Mobile communicationmeans, comprising at least one filter structure having a first signalline, a second signal line, a third signal line and a fourth signalline, said first and third signal lines defining an input port and saidsecond and fourth signal lines defining an output port of a section ofsaid filter structure, said section being defined by a first bulkacoustic wave resonator (A) which is connected between said first signalline and said second signal line, a second bulk acoustic wave resonator(G) which is connected between said third signal line and said fourthsignal line, a third bulk acoustic wave resonator (C) which is connectedbetween said first signal line and said fourth signal line, and a fourthbulk acoustic wave resonator (E) which is connected between said secondsignal line and said third signal line; wherein at least one furthersection of first, second, third and fourth acoustic wave resonators (B,H, F, D) is provided, wherein the input port of each said furthersection is connected with the output port of a preceding section to forma multiple-section filter structure characterized in that a frequencypulling factor defined δ of at least one of said acoustic waveresonators is non-zero, said frequency pulling factor δ being defined by$\delta = {2{( {\frac{f_{r1}}{f_{0}} - 1} ) \cdot \frac{1}{k^{2}}}}$for first and second bulk acoustic wave resonators having a resonantfrequency f_(r1), and$\delta = {2{( {1 - \frac{f_{a2}}{f_{0}}} ) \cdot \frac{1}{k^{2}}}}$for third and fourth bulk acoustic wave resonators having ananti-resonant frequency f_(a2), f₀ being the centre frequency of thefilter structure and k the coupling factor of respective resonators. 9.Layout for a filter structure according to any of claims 1 to 7,characterized in that it is realized essentially in a plane bytransferring crossovers of connecting lines to input and output ports tounfold the structure.